RAR archive of 1.59 MB for Apple board number 820-2901, with nothing else stated in the name: no model, revision or description of contents. The 820 prefix is Apple's logic board series, so the reference is the number on the PCB itself. Typically such a package carries the schematic as a PDF, used to follow power rails, understand the SMC and power sequencing and identify parts by designator. Extract with WinRAR or 7-Zip and confirm the revision before trusting component values or rail names.
The button opens the matching file on RM Schematics, or a source page if it is not catalogued there yet. The original attachment is from @Mac_Shematic_Santale. Enginerys does not host this file.
How to open a .rar file
Compressed archive — the usual wrapper for a schematic + boardview pair or a BIOS + EC set.